Vibration dampers are used to reduce the vibration transmitted from a high-vacuum pump, for example, a turbomolecular pump, to apparatus to be evacuated during a pumping operation. Vibration dampers are particularly advantageous when vacuum pumps are used to evacuate apparatus which is sensitive to mechanical vibration. For example, vibrations transmitted to an evacuated Scanning Electron Microscope could lead to inaccuracies in measurements being taken by the microscope, and vibrations transmitted to a process tool could cause anomalies in products being manufactured within.
With reference to FIG. 2, a vibration damper 0 is typically connected between the fluid exhaust 9 of the apparatus 7 to be evacuated and the fluid inlet 10 of the vacuum pump 8. FIG. 1 illustrates the configuration of a known vibration damper 0 in more detail. The damper 0 includes two flanges 2,3 each welded to a respective end of a steel bellows 4. Each flange 2, 3 has an aperture 2a, 2b formed therein, the apertures 2a, 2b being axially aligned. The bellows 4 defines a flow path 4a through the damper 0 for fluid pumped from the apparatus 7 by the pump 8.
A mechanical support 5 is provided to prevent the bellows 4 collapsing under compression when the fluid in the flow path 4a is at low pressure that is, under vacuum and external forces due to atmospheric pressure act to compress the damper 0. In the example shown in FIG. 1, the mechanical support 5 is provided by an elastomeric cylinder surrounding the bellows 4 between the flanges 2, 3.
Interlinking members 6a, 6b are provided to prevent the bellows 4 from extending under the weight of the pump 8 suspended from flange 3 when the apparatus is not under vacuum. In the example shown in FIG. 1, member 6a is in the form of a V-shaped metallic strap welded to the top of flange 2, and member 6b is in the form of a similar strap welded to the bottom of flange 3 so that the members 6a, 6b are linked. In the illustrated example, when the damper 0 is not connected to any other components, the members 6a, 6b are not in contact.
The parameter governing transmission of vibration from the pump 8 to the apparatus 7 is the stiffness (k) of the damper 0. Bellows 4 are typically chosen to define the flow path 4a in view of their low inherent axial stiffness, so as to cause minimal transmission of vibration to the apparatus 7. The interlinking straps 6a, 6b are not in contact when under vacuum conditions, where the damper 0 experiences compressive loading. Consequently, the primary route for vibration transmission is through the mechanical support 5.
Vibration dampers positioned between the apparatus 7 and the pump 8, as shown in FIG. 2, are subject to a large static force acting on the lower end of pump 8 which acts to force the pump 8 towards the apparatus 7, this force being associated with the pressure difference between atmospheric and vacuum conditions. This force must be borne by the mechanical support 5 of the damper 0. However, when known elastomeric mechanical supports 5 are exposed to such loading conditions, their hyperelasticity, reflected in a non-linear progressive stiffness characteristic, causes them to become increasingly stiff or rigid. Under such compressive loading conditions, the increased rigidity enhances transmission of vibration to the apparatus 7 rather than reducing it. Furthermore, known elastomeric mechanical supports 5 typically experience failure in a buckling mode.
It is an object of the present invention to provide a vibration damper that substantially reduces the problems associated with these prior art vibration dampers.